You've left out a lot of detail needed to help answer your question.
I'm assuming you are doing thermal oxide in silicon over a doped region
as the conductor. In this case, the surrounding silicon to the doped
region will have a substantial effect, depending on how highly doped it
is in the first place. There will be charge leakage through that
silicon, unless you form a p-n diode and reverse bias it - in which case
bipolar operation is out.
You don't indicate where the membrane is in relation to all this and how
it is driven.
--
Tom Rust
Nanochip Inc